Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 6067−6072
pubs.acs.org/JPCL
Spin−Orbit State-Selective C−I Dissociation Dynamics of the CH3I+ X̃ Electronic State Induced by Intense Few-Cycle Laser Fields Zhengrong Wei,† Jialin Li,† Soo Teck See,† and Zhi-Heng Loh*,†,‡ †
Division of Chemistry and Biological Chemistry and Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ‡ Centre for Optical Fibre Technology, The Photonics Institute, Nanyang Technological University, Singapore 639798, Singapore S Supporting Information *
ABSTRACT: Studies of ultrafast molecular dynamics induced by intense laser fields can reveal new approaches to manipulating chemical reactions in the strong-field regime. Here, we show that intense few-cycle laser pulses can induce the spin−orbit state-selective C−I dissociation of the iodomethane cation (CH3I+) in the X̃ electronic state. Irradiation of CH3I by 6 fs laser pulses with peak intensities of 1.9 × 1014 W/cm2 followed by femtosecond extreme ultraviolet probing of the iodine 4d core-level transitions reveals dissociation of the CH3I+ X̃ 2E1/2 state with a time constant of 0.76 ± 0.16 ps. By contrast, the X̃ 2E3/2 spin−orbit ground state does not exhibit any appreciable dissociation on the picosecond time scale. The observed spin−orbit state-selective dissociation of the X̃ state is rationalized in terms of the laser-induced coupling to the à state. Our results suggest that the intense-laser control of photodissociation channels can be potentially extended to spin−orbit split states. presence of intense laser fields, dissociative ionization of CH3I+ and multiple Coulomb explosion pathways of CH3In+ (n > 1) are revealed by velocity map imaging of various fragments and measurements of the kinetic energy release for different dissociation channels.35−37 In the present study, we focus on elucidating the ultrafast CH3I dissociation dynamics in the presence of intense laser fields. We observe spin−orbit stateselective dissociation of the CH3I+ X̃ state; while the 2E3/2 spin−orbit ground state is stable with respect to C−I dissociation, the 2E1/2 spin−orbit excited state dissociates on a time scale of 0.76 ± 0.16 ps. In addition, dissociation of the CH3I+ à 2A1 state occurs with a time constant of 86 ± 11 fs. The observed dissociation dynamics are rationalized in terms of the coupling between the X̃ and à potential energy surfaces induced by the intense laser field. Finally, our results suggest that both the CH3I+ X̃ 2E3/2 and 2E1/2 states undergo an ultrafast structural rearrangement on a time scale of 75 ± 8 fs immediately upon their formation. Strong-field ionization is effected by the irradiation of neutral CH3I molecules by intense laser pulses with a carrier wavelength of 786 nm (1.58 eV photon energy) and a pulse duration of 5.6 fs fwhm. The few-cycle pulse, with pulse energy of 0.17 mJ, is loosely focused by a 1 m focal length spherical mirror to a 100 μm beam waist to yield a peak intensity of 1.9 × 1014 W/cm2. The sample target comprises 14 mbar of CH3I in a 3 mm path length quasi-static gas cell that is heated to 353 K. Details of the experimental apparatus can be found in ref 38. The XUV differential absorption spectra collected as a function
A
dvances in tabletop femtosecond laser technologies enable routine access to laser pulses with peak intensities exceeding ∼1 PW/cm2.1 From the chemical perspective, the high electric-field strengths that are furnished by these intense laser pulses can be harnessed to control the outcomes of chemical reactions.2−9 For example, branching ratios of nonadiabatic photodissociation dynamics have been manipulated via the dynamic Stark effect3 and light-induced conical intersections.8 The realization of various strong-field control schemes requires an understanding of how molecules interact with laser fields in the nonperturbative, strong-field regime. Early investigations of strong-field laser−molecule interactions focused on the simplest molecule, H2+, whose dissociation in intense laser fields has been extensively studied by both experiment and theory.10−15 These studies elucidate new phenomena such as bond softening, bond hardening, and Coulomb explosion. Beyond H2+, efforts have been undertaken to unravel the interaction of strong laser fields with multielectron molecules.16−23 Compared to the case of H2+, these studies reveal complex angle-dependent ionization yields,24,25 participation of multiple channels in strong-field ionization,26,27 multielectron nonadiabatic dynamics,22,23 and a variety of molecular dynamics.17,28 Recent investigations have also unraveled the time scales for strong-field dissociative ionization,29−32 a key parameter in the understanding of ultrafast photochemical reactions.33 Here, we employ femtosecond extreme ultraviolet (XUV) absorption spectroscopy34 to investigate the strong-fieldinduced dissociation dynamics of iodomethane (CH3I). As a prototypical polyatomic molecule, the photodissociation dynamics of its neutral excited states in the weak-field regime has been the subject of numerous investigations.2 In the © XXXX American Chemical Society
Received: November 14, 2017 Accepted: November 30, 2017 Published: November 30, 2017 6067
DOI: 10.1021/acs.jpclett.7b03022 J. Phys. Chem. Lett. 2017, 8, 6067−6072
Letter
The Journal of Physical Chemistry Letters
Table 1. Observed XUV Absorption Peaks for CH3I+ (X̃ ), I, I+, and CH3I (X̃ ); Their Assignments to the I 4d Core-Level Transitions; and the Transition Energies Obtained from the Literature
of time delay is shown in Figure 1a. The negative differential absorption signal (ΔA) that is observed in the probe photon
species CH3I+ (X̃ )
peak position (eV)
peak position, lit. (eV)
transition −1 E1/2 → 4d5/2
46.6
46.45
a
2
47.0
47.08a
2
48.1
48.19a
2
48.7
48.82a
2
52.6
−
2
−1 E 3/2 → 4d5/2
−1 E1/2 → 4d3/2
−1 E 3/2 → 4d3/2
−1 −1 * E 3/2 → 4d5/2 e3/2σC − I
−1 −1 * E1/2 → 4d5/2 e1/2σC − I
2
54.2
−
−1 −1 * E 3/2 → 4d3/2 e3/2σC − I
2
−1 −1 * E1/2 → 4d3/2 e1/2σC − I
2
CH3I (X̃ )
Figure 1. (a) Differential XUV absorption spectra collected as a function of pump−probe time delay. (b) Fit of the differential XUV absorption spectrum at 2 ps to multiple Gaussians. The red symbols are the data, and the solid black line is the fit. Contributions from absorption of CH3I+, neutral CH3I, neutral I, and singly charged I+ are color-coded.
I (2P3/2)
I+ (3P2)
energy range of ∼50−52 eV is due to bleaching of the CH3I ground-state 1A1 → 4d−1 * transition39 (j = 5/2 and 3/2) j σC−I that accompanies the depletion of the neutral CH3I population by strong-field ionization. On the other hand, the positive ΔA signal in the 46−49 eV range can be assigned to the groundstate CH3I+ parent ion and the atomic I (2P3/2) and I+ (3P2) photoproducts produced by the intense laser−molecule interaction. Pronounced modulations of the transition energies about the neutral CH3I depletion region and, to a lesser extent, about the CH3I+ parent ion peak at 47.0 eV (see below for assignment) originate from coherent vibrational motions launched by the intense laser field.38 The ΔA spectrum collected at 2 ps time delay and the fits to the XUV transitions of the various species are shown in Figure 1b. The peak positions and spectral assignments of the observed XUV transitions are summarized in Table 1. In assigning the differential XUV absorption spectrum, our strategy is to consider the species that are most likely to form under our experimental conditions. We start by identifying the transitions that arise from the depletion of the neutral CH3I and the formation of the CH3I+ parent ion. Next, we identify the absorption peaks that are due to atomic I (2P3/2) and I+ (3P2), which are produced as a result of dissociation in the low-lying (X̃ and à ) electronic states of CH3I+.41 With this approach, we are able to account for all the features that appear in the differential XUV absorption spectrum. Higher charged states of CH3In+ and In+ (n ≥ 2) do not seem to be present in detectable quantities. For example, the I2+ (4S3/2) ground state possesses a pronounced absorption peak at 48.93 eV,40 which is not observed in the differential XUV absorption spectrum. Finally, we do not consider the formation of atomic I 2P1/2 species, which originates from the dissociation of the B̃ state that is located ∼5−6 eV above the X̃ state of CH3I+ in the Franck−Condon region.41 Given our laser peak intensity, we
−1 * A1 → 4d5/2 σC − I
50.6
50.62b
1
52.3
52.34b
1
46.0
45.94c
2
47.6
47.64c
2
47.3
47.32c
3
49.0
48.95c
3
49.2
49.17c
3
−1 * A1 → 4d3/2 σC − I
P 3/2 → 2D5/2 P 3/2 → 2D3/2
D2 → 3D3
P 2 → 3P 2 P 2 → 3D1
a Calculated based on literature values for the valence and I 4d core level binding energies of CH3I (see the Supporting Information). bRef 39. cRef 40.
do not expect any appreciable population of the B̃ state by strong-field ionization. Assignment of the features observed in the positive ΔA spectral region requires knowledge of the transition energies of the CH3I+ ion. These transition energies can be estimated within the framework of the single-particle picture,42 which considers XUV transition to involve the promotion of an electron from the I 4d orbital to the single vacancy in the highest occupied molecular orbital (HOMO) of e symmetry. 2 Using the original atomic I 4d−1 j notation, these are the E1/2 → 2 −1 2 −1 2 −1 −1 4d5/2 , E3/2 → 4d5/2 , E1/2 → 4d3/2 , and E3/2 → 4d3/2 transitions, whose transition energies are calculated to be 46.45, 47.08, 48.19, and 48.82 eV, respectively (see the Supporting Information). These predicted transition energies agree well with the peaks observed at 46.6, 47.0, 48.1, and 48.7 eV. Aside from the four peaks that can be assigned to the CH3I+ parent ion, a peak at 46.0 eV is also present in the ΔA spectrum. On the basis of its absence in the ΔA spectrum at earlier time delays, it is assigned to the 2P3/2 → 2D5/2 transition of the atomic I 2P3/2 state, in good agreement with the literature value of 45.94 eV.40 The accompanying 2P3/2 → 2D3/2 transition, reported to be at 47.64 eV, is observed at 47.6 eV.40,43 It is noteworthy that the I 2P1/2 → 2D3/2 transition at 46.70 eV is not observed in our experiment, which suggests that the strong-field dissociative ionization of CH3I, under our experimental conditions, yields neutral atomic I only in the 2 P3/2 state. In addition to neutral atomic I (2P3/2), a shoulder at 47.3 eV that is evident in the ΔA spectrum collected at 2 ps time delay (Figure 1b) can be assigned to the 3P2 → 3D3 6068
DOI: 10.1021/acs.jpclett.7b03022 J. Phys. Chem. Lett. 2017, 8, 6067−6072
Letter
The Journal of Physical Chemistry Letters transition of I+, in good agreement with the transition energy of 47.32 eV that is reported in the literature.40 Other transitions for the I+ 3P2 state with significant oscillator strengths, such as the 3P2 → 3P2 (48.95 eV) and 3P2 → 3D1 (49.17 eV) transitions,40 are also observed at 49.0 and 49.2 eV, respectively. XUV transitions with large oscillator strengths that originate from the 3P1 and 3P0 states of I+, e.g., the 3P1 → 3 D2 and the 3P0 → 3P1 transitions at 47.80 and 47.15 eV, respectively, are notably absent. This observation suggests that the I+ fragment is produced solely in the 3P2 ground state during the dissociative ionization of CH3I. The negative peaks at 50.6 and 52.3 eV in the ΔA spectra, which arise from the I 4dj → σ* transitions (j = 5/2 and 3/2) of depleted CH3I, have relative amplitudes that deviate significantly from the 1.5:1 ratio that is expected based on the (2j + 1)-degeneracy of the I 4d core-hole states. This observation suggests that a product of strong-field ionization of CH3I exhibits absorption features that partially overlap with the I 4d3/2 → σ* transition of CH3I. Following previous work on the XUV probing of strong-field-ionized Br2,44 we assign the features with positive ΔA to the I 4dj → σ* transitions (j = 5/2 and 3/2) of CH3I+; curve fitting yields absorption peaks at 52.6 and 54.2 eV and a relative ratio of 1.7 ± 0.1:1 for the peak areas. The blueshift of the I 4dj → σ* transition energies by 2.0 eV upon ionization points to the preferential stabilization of the I 4d core level over the valence σ* level upon the removal of an electron from the outermost valence e orbital of CH3I. Note that this curve fitting takes into account the negative ΔA signal that arises from the perturbed free-induction decay (PFID) of the I 4d−1 5/2 → 6pe transition at 54.3 eV; previous experiments on atomic species show that measuring the temporal evolution of the PFID feature yields autoionization lifetimes.45,46 Figure 2 shows the temporal evolution of the ΔA signals collected for the X̃ + 2E3/2 and 2E1/2 states of the CH3I+ parent 2 −1 ion at the 2E3/2 → 4d−1 5/2 (47.0 eV, Figure 2a) and E1/2 → 4d3/2 (48.1 eV, Figure 2b) transitions, respectively, along with the growth of the neutral I (2P3/2) and the singly charged I+ (3P2) species observed at their respective 2P3/2 → 2D5/2 (46.0 eV, Figure 2c) and 2P2 → 3D3 (47.3 eV, Figure 2d) transitions. The positive ΔA signals for both the X̃ 2E3/2 and 2E1/2 states of CH3I+ appear within the
